Vinyl aromatic monomers, such as styrene, divinylbenzene and the like, produced by conventional processes contain byproducts and impurities. These undesired impurities must be removed from the desired monomer by separation and purification processes such as distillation in order for the monomer be suitable for further industrial applications. The elevated temperatures encountered during distillation causes thermal polymerization and formation of unwanted polymer. The polymer formation can be reduced by the use of a polymerization inhibitor.
In a typical purification process, the impure vinyl aromatic monomer to be distilled is mixed with a polymerization inhibitor before being subjected to distillation conditions in the distillation system. However, a significant amount of undesired polymer is often formed in the distillation system which substantially reduces the yield of high purity monomer obtained as well as causing severe and costly operational difficulties in the purification process. Still worse, occasionally, complete premature polymerization of the vinyl aromatic monomer occurs causing considerable economic loss. A typical distillation system is described in detail in U.S. Pat. Nos. 4,252,615 and 4,341,600, the relevant parts of which are incorporated herein by reference.
To prevent premature polymerization of vinyl aromatic monomers during the distillation purification process, various compounds have been used as polymerization inhibitors. Sulfur was widely employed in the past for this purpose. However, more recently, a number of organic compounds have been disclosed and used as substitutes for sulfur as polymerization inhibitors for such monomers with mixed success.
U.S. Pat. No. 4,086,147 discloses 2-nitro-p-cresol as a polymerization inhibitor. U.S. Pat. Nos. 4,105,506 and 4,252,615 disclose 2,6-dinitro-p-cresol as such a polymerization inhibitor. U.S. Pat. Nos. 4,132,602 and 4,132,603 disclose the use of a halogenated aromatic nitro compound as a polymerization inhibitor for use during the distillation of vinyl aromatic compounds. However, in each of these cases, the organic compounds have relatively weak activity and must be used at fairly high concentrations for any significant inhibition activity especially when higher distillation temperatures are involved. Additionally, the relatively high toxicity of these aromatic nitro compounds makes them far from attractive as a practical solution to the premature polymerization problem encountered with vinyl aromatic monomers during their purification and distillation.
U.S. Pat. Nos. 3,988,212 and 4,341,600 disclose the use of N-nitroso-diphenylamine combined with dinitrocresol derivatives for inhibiting the polymerization of vinyl aromatic compounds under vacuum distillation conditions. U.S. Pat. No. 4,466,904 discloses the use of phenothiazine, 4-tert-butylcatechol and 2,6-dinitro-p-cresol as a polymerization inhibitor system in the presence of oxygen during heating of a vinyl aromatic monomer. U.S. Pat. No. 4,468,343 discloses the use of 2,6-dinitro-p-cresol and either a phenylenediamine or 4-tert-butylcatechol in the presence of oxygen to prevent premature polymerization of vinyl aromatic compounds during heating. European patent application EP 0240297 A1 teaches the use of a substituted hydroxylamine and a dinitrophenol to inhibit the premature polymerization of a vinyl aromatic compound at elevated temperatures as during distillation. However, in each of these cases the effectiveness of the polymerization inhibitor mixture is oxygen dependent. This results in inconsistent inhibition due to a variable and unpredictable distribution of air (oxygen) throughout the distillation system. Additionally, there is the greatly increased possible explosion safety hazard which may occur because of the presence of air in the system. It is clear that there still exists a great need for a stable polymerization inhibitor system of relatively low toxicity which will effectively and safely prevent the premature polymerization of vinyl aromatic compounds during distillation particularly in the absence of oxygen (air).
U.S. Pat. No. 3,733,326 discloses the polymerization inhibition of vinyl monomers by use of free radical precursors. Soviet Patent No. 1,027,150 teaches the stabilization of styrene by using a nitroxyl radical. Soviet Patent No. 1,139,722 discloses the use of a bis-nitroxyl compound as a thermal polymerization inhibitor for styrene. Japanese Hei 1-165534 teaches the use of 1-piperidyloxy derivatives as polymerization inhibitors for styrene. Soviet Patent No. 1,558,888 discloses the polymerization inhibition of styrene by a nitroxyl radical. The inhibition of styrene polymerization by selected hindered amine nitroxyl derivatives is also discussed by Y. Miura et al., Makro mol. Chem. 1972, 160, 243; by M. D. Golf'fein et al., Vysokomol. soyed 1975, A17 (8), 1671 translated in Polymer Science (USSR), 1975, A17 (8), 1919; and by G. Moad et al., Polymer Bull. 1982, 6, 589. The Moad reference teaches the use of 1-oxyl-2,2,6,6-tetramethylpiperidine and 2-oxyl-1,1,3,3-tetramethylisoindoline, but do not suggest that the mixtures prepared in their experimental section are per se useful as an inhibitor when added to fresh styrene or any other aromatic vinyl monomer to prevent the premature polymerization of said monomer. U.S. Pat. No. 5,254,760 discloses the use of stable hindered nitroxyl compounds combined with aromatic nitro compounds to prevent premature polymerization of vinyl aromatic compounds during distillation and purification processes.